When a hearing aid is subject to an ultrasound field in the environment, very annoying side effects may be experienced by the hearing aid user even if the ultrasound is at a frequency far above the audio range of the human ear. One side effect is possible saturation of microphones and the analogue to digital converter of the apparatus. The saturation is a result of the very powerful sound pressures which are not uncommon in connection with ultrasound devices like automatic door openers or alarm systems. Another side effect is caused by the nonlinearity whereby the sound waves will often convolve down and result in the generation of sound in the audio frequency range where people can hear it as noise. Also headsets or other listening devices having microphones may be adversely effected by the presence of ultrasound in the environment.
One way to reduce this effect is to use a ¼ wave resonator or other filter in the inlets before the microphones which reduces or removes the ultrasound before it reaches the microphone. This arrangement can be very costly because each microphone inlet must be modified to accommodate either the filter or the ¼ wave resonator, and further this complicates the production. Also filters may not in all instances be sufficient, in order to remove all harmful effects of the ultrasound sound field.